This invention relates generally to boron carbide-gadolinium oxide, boron carbide-gadolinium, and gadolinium oxide-gadolinium compositions of matter and more particularly to boron carbide-gadolinium oxide ceramic composites and boron carbide-gadolinium and gadolinium oxide-gadolinium cermets or metal-matrix composites.
U.S. Pat. No. 4,605,440 by Halverson, Pyzik and Aksay, U.S. Pat. No. 4,704,250 by Cline and Fulton, and U.S. Pat. No. 4,718,941 by Halverson and Landingham all pertain to boron carbide-reactive metal composites and their manufacture. However, these patents do not show specific boron carbide-gadolinium compositions or methods for producing directly usable consolidated bodies thereof. U.S. Pat. Nos. 4,826,630 and 4,474,728 by Carlson and Radford, U.S. Pat. No. 4,744,922 by Blakely and Shaffer, U.S. Pat. No. 4,671,927 by Alsop, and U.S. Pat. Nos. 4,657,876 and 4,636,480 by Hillig describe various neutron absorbing materials.
The field of nuclear physics has matured and created a revolutionary impact on modern history. The applications of research in neutron-induced reactions appear both in other areas of fundamental research and in the practical areas of nuclear energy production.
The ultimate fate of a free neutron liberated through some reaction is either absorption by a nucleus, or transformation by beta decay. The latter process is so weak as to be negligible for practical applications. By a wide margin, the most important absorption process in a non-fissile nucleus is radiative capture.
Neutron capture can occur over nine orders of energy magnitude, slow or thermal neutrons from as low as 10.sup.-2 eV to fast neutrons as high as 14 MeV. Different atomic mechanisms are associated with the radiative capture of each. Neutron capture processes have lifetimes ranging from 10.sup.-22 seconds (thermal capture) to as long as 10.sup.-15 seconds (fast capture).
For practical applications involving slow neutrons, it is sufficient to work with the statistical theory of radiative capture. This topic is highly complex; however, qualitative consideration of materials can be made by initially examining their average capture cross section properties.
The nuclear cross section of a material is a measure of the probability of a particular process. In the case of radioative capture, the capture cross section is expressed as .sigma., and is the effective target area of the nucleus with which a neutron must interact to produce a given reaction. The unit of .sigma. is the barn (1 barn=10.sup.-24 cm.sup.2). Absorption cross sections for thermal neutrons range from 4.6.times.10.sup.-4 barn for deuterium to 3.3.times.10.sup.6 barns for xenon.sup.135.
To protect nuclear reactor operating personnel against damaging biological effects of neutrons and gamma rays, shielding is required around nuclear reactors. Neutron and gamma ray fluxes in the range of 10.sup.13 to 10.sup.14 must be attenuated to 10.sup.3 particles/cm.sup.2 /sec to meet tolerance radiation levels.
To attenuate gamma rays, which interact primarily with the orbital electrons of atoms, a material with high atomic number containing a high density of these electrons is required. Examples are lead, tungsten, depleted uranium, or concrete containing high-Z elements in the form of scrap or heavy ore.
To attenuate neutrons, they must be slowed down and then absorbed. Hydrogenous materials such as water, concrete, or polyethylene are excellent moderators. The slowed neutrons must be absorbed without producing high-energy-capture gamma rays. This has historically been accomplished by using boron.sup.10 ; however, even with boron.sup.10 some gamma shielding outside the neutron shield is generally required.
When a neutron is absorbed by the nucleus of an atom an exothermic process results and the compound nucleus reaches an excited energy state between 4 and 10 MeV, as determined by the center-of-mass kinetic energy and the rest-mass energy difference of the final and initial nuclides. This state decays by the emission of electromagnetic (gamma) radiation, leaving the compound nucleus in a lower energy state. Subsequent radiative decay of this and lower energy states, i.e., a cascade of gamma rays, leaves the compound nucleus in its ground state, which may, or may not, be stable against alpha or beta decay (daughter products).
The inherent atomic processes associated with the radiative capture of neutrons results in exothermic reactions that typically prohibit the use of hydrogenous materials because of their low-temperature phase changes; e.g., water boils at 100.degree. C., polyethylene softens near 87.degree. C.
Boron metal also has its drawbacks. It corrodes easily and is physically unstable under irradiation. Alloying to overcome these problems merely reduces the boron content of the absorbing material. Because of these concerns, boron carbide has been used extensively as a neutron absorbing material in various types of nuclear reactors for several decades.
Boron carbide exists as a homogeneous range of boron and carbon compositions between 9 and 24 at. % C. The most common stoichiometries being B.sub.4 C (B.sub.12 C.sub.3) and B.sub.13 C.sub.2, both of which are boron rich. Richer boron stoichiometries, B.sub.8 C and B.sub.25 C, are also known to exist; however, these are less favored thermodynamically. The high boron content and refractory nature of B.sub.4 C (melting point .apprxeq.2350.degree. C.) made it a choice candidate for high temperature neutron absorbing reactions.
The ideal neutron absorbing material would be light weight, refractory, not impart long-lived daughter products, be thermally shock resistant, of low density yet not too porous, be resistant to corrosion and oxidation, have high fracture toughness and high strength, and not promulgate dust on delivery or while in use. Low cost, obviously, would be another attractive advantage.
Boron carbide is refractory, has a specific gravity of 2.52, a modulus of rupture .apprxeq.300 MPa, and can be hot pressed into fully dense bodies. Boron carbide displays low fracture toughness, and also rapidly oxidizes above 800.degree. C. In addition, boron carbide's thermal shock resistance is poor.
One way to increase the fracture toughness and thermal shock resistance of B.sub.4 C is by the addition of a metal phase, e.g., B.sub.4 C-metal cermets or metal-matrix composites. A cermet is defined as a ceramic-metal composite such that the final microstructure is .gtoreq.50 vol. % ceramic phases. A metal matrix composite is defined as a ceramic-metal composite such that the final microstructure is &lt;50 vol. % ceramic phases. The ceramic phases can be the initial starting ceramic materials or reaction products that result from chemical reactions between two ceramic phases or between ceramic and metal phases.
Another way to increase the fracture toughness and strength of B.sub.4 C is through the introduction of another ceramic phase, e.g., B.sub.4 C-Al.sub.2 O.sub.3, B.sub.4 C-TiB.sub.2, and B.sub.4 C-SiC composites. Although large increases in fracture toughness and strength are generally obtained with the addition of a metal phase, the introduction of another ceramic phase can increase toughness while maintaining the refractory nature of the composite.
One of the most appropriate metal phases to consider for the absorption of neutrons is gadolinium, Gd. This metal also exists in the form of a stable oxide known as gadolinium oxide, Gd.sub.2 O.sub.3. Gadolinium has the highest nuclear capture cross section of any element known, .sigma..apprxeq.40,000 barns, compared to B.sup.10 with a .sigma..apprxeq.4,000 barns.
Gadolinium offers mechanical and physical properties conducive to fabricating B.sub.4 C-Gd cermets or metal-matrix composites, according to the invention, that approach "ideal" neutron absorbing material conditions. For example, Gd is used as a burnable poison in shields and control rods in nuclear reactors. It has a melting point of 1313.degree. C., a boiling point of .apprxeq.3000.degree. C., and an .alpha..fwdarw..beta. transformation temperature of 1235.degree. C. Gadolinium tarnishes slightly in air at room temperature; however, even at 1000.degree. C. the oxidation rate is slow because of the formation of the tightly adhering oxide on the surface. It does not react with cold or hot water, but will react vigorously with dilute acids.
Gadolinium has the following mechanical properties: Tensile strength .apprxeq.122 MPa, yield strength .apprxeq.17 MPa, elongation .apprxeq.47%, reduction in area .apprxeq.58%, and an elastic modulus .apprxeq.56 GPa. It also has a thermal expansion coefficient of .apprxeq.9.times.10.sup.-6 /.degree. C.
Gadolinium's very low modulus of elasticity indicates it should be substantially more resistant to thermal shock than B.sub.4 C. By forming a B.sub.4 C-Gd composite, according to the invention, it should be possible to obtain a refractory body with very high neutron absorbing capability and good thermal shock resistance. Because the specific gravity of Gd is 7.90, the addition of B.sub.4 C will also reduce the composite's weight substantially. Also, according to the invention, the reactions between Gd and B.sub.4 C during processing will introduce other ceramic phases into the composite resulting in a higher overall fracture toughness.
According to the invention, similar material properties should be obtained by combining Gd.sub.2 O.sub.3 and B.sub.4 C, or Gd.sub.2 O.sub.3 and Gd, to form ceramic-ceramic composites, cermets, or metal-matrix composites. For example, Gd.sub.2 O.sub.3 has a specific gravity of 7.41 and a elastic modulus of .apprxeq.130 GPa.
Accordingly, it is an object of the present invention to provide boron-carbide-gadolinium cermet compositions, boron-carbide-gadolinium metal-matrix compositions, boron-carbide-gadolinium-oxide compositions, gadolinium-oxide-gadolinium cermet compositions, and gadolinium-oxide-gadolinium metal-matrix compositions.
It is also an object of the invention to provide methods for forming boron-carbide-gadolinium cermet compositions, boron-carbide-gadolinium metal-matrix compositions, boron-carbide-gadolinium-oxide compositions, gadolinium-oxide-gadolinium cermet compositions, and gadolinium-oxide-gadolinium metal-matrix compositions.
It is another object of the invention to provide boron-carbide-gadolinium cermet compositions, boron-carbide-gadolinium metal-matrix compositions, boron-carbide-gadolinium-oxide compositions, gadolinium-oxide-gadolinium cermet compositions, and gadolinium-oxide-gadolinium metal-matrix compositions with refractory microstructures, and methods for forming same.
It is a further object of the invention to provide boron-carbide-gadolinium cermet compositions, boron-carbide-gadolinium metal-matrix compositions, boron-carbide-gadolinium-oxide compositions, gadolinium-oxide-gadolinium cermet compositions, and gadolinium-oxide-gadolinium metal-matrix compositions which are fully dense, and methods for forming same.
It is another object of the invention to provide articles of manufacture made from boron-carbide-gadolinium cermet compositions, boron-carbide-gadolinium metal-matrix compositions, boron-carbide-gadolinium-oxide compositions, gadolinium-oxide-gadolinium cermet compositions, and gadolinium-oxide-gadolinium metal-matrix compositions.
It is also an object of the invention to provide methods for making boron-carbide-gadolinium cermet compositions, boron-carbide-gadolinium metal-matrix compositions, boron-carbide-gadolinium-oxide compositions, gadolinium-oxide-gadolinium cermet compositions, and gadolinium-oxide-gadolinium metal-matrix compositions, and articles of manufacture thereof at relatively low cost.